Method and apparatus for controlling ringing voltage

ABSTRACT

A method for controlling power dissipated in a subscriber line interface circuit includes measuring a power parameter of the subscriber line interface circuit during a ringing cycle, comparing the measured power parameter to a target power parameter and adjusting at least one ringing parameter of a ringing signal generated by the subscriber line interface circuit based on the comparison. A line card includes a subscriber line interface circuit operable to generate a ringing signal, and a subscriber line audio-processing circuit operable to measure a power parameter of the subscriber line interface circuit during a ringing cycle, compare the measured power parameter to a target power parameter, and adjust at least one ringing parameter of the ringing signal based on the comparison.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND

The disclosed subject matter relates generally to telecommunicationsand, more particularly, to a method and apparatus for controllingringing voltage.

In communications systems, particularly telephony, it is common practiceto transmit signals between a subscriber station and a central switchingoffice via a two-wire bidirectional communication channel. A line cardgenerally connects the subscriber station to the central switchingoffice. A line card typically includes at least one subscriber lineinterface circuit (SLIC) as well as a subscriber line audio-processingcircuit (SLAC). The functions of the line card include range fromsupplying talk battery to performing wake-up sequences of circuits toallow communications to take place.

Subscriber line interface circuits (SLICs) have been developed toprovide an interface between a low voltage signal path in a telephonecentral office and a high-voltage telephone subscriber line. The SLICprovides functions such as off hook detection, ringing signalgeneration, and battery feed to the subscriber line. The subscriber lineconsists of a telephone transmission line, including two conductorsreferred to as A and B or tip and ring, and the subscriber telephoneequipment coupled across the tip and ring conductors (i.e., the load).The subscriber line and the subscriber telephone equipment are alsoreferred to as a subscriber loop.

The SLIC provides power from the telephone central office to thesubscriber line in response to a received battery voltage. The batteryvoltage is a DC voltage supplied to the SLIC to power the SLIC and thesubscriber line. A typical value of the battery voltage is −48 VDC. Thebattery voltage has a value generally in the range −20 to −60 VDC. TheSLIC supplies a DC current at the battery voltage to the subscriberline. Superimposed on the DC current are AC signals of audio frequencyby which information is conveyed between the subscriber and the centraloffice. The battery voltage is generated at the central office, eitherby a depletable energy storage device such as a battery or by a DCgenerator, for supply to the SLIC. In a central office, one battery orDC generator supplies the battery voltage to many SLICs and theirassociated subscriber loops.

In many modern applications, a SLIC is located remote from the centraloffice, relatively close to the subscriber telephone equipment andcoupled to the subscriber telephone equipment by a relatively shortsubscriber line. For example, in fiber in the loop (FITL) applications,the SLIC is located in the same city neighborhood as the subscribertelephone equipment and is coupled to the subscriber telephone equipmentby tip and ring conductors no more than a few hundred feet in length.The SLIC or an associated circuit receives optical signals from thecentral office over an optical fiber and converts the optical signals toAC electrical signals. In response to the electrical signals, the SLICsupplies AC signals of audio frequency, along with DC power from thebattery, to the subscriber line. In such applications, where the SLICand battery are remote from the central office, one battery or batteryvoltage generator may supply the battery voltage to only one or a fewSLICs and their associated subscriber loops.

In applications where a SLIC is used in a central office and there areshort lines, the loop voltage drop is low, resulting in a high voltagedrop and consequently high power dissipation in the SLIC. In a denseenvironment of such devices, related heating can cause failures. Inaddition, with SLICs having resistive feed characteristics used in shortlines, current is higher, further compounding this problem.

Newer generation chipsets are designed to operate in high density linecard applications. The limited board space available in such line cardsconstrains the size of the package that is available for the deviceslike the SLIC and the SLAC. Another consideration that also reduces thesize of the package is the desire to have lower per line cost for theline card. In general the smaller packages do not present a problem whenthe associated device does not generate significant heat. However forsilicon devices, like the SLIC, that interface with the telephone line,the reduced silicon die size and the reduced package size coupled withthe requirement of having to drive out heavy duty ringing signals to thetelephony equipment present a challenging design problem with respect toheat generation.

In general, the ringing state is a challenging state in terms of powerdissipation for the SLIC device. In a ringing state, the SLIC makes useof all of the available battery sources to drive out the maximum ringingsignal. The rationale behind driving out the maximum possible ringing isthat the SLIC needs to apply upwards of 40 Vrms for the ringing signalat the longest loop (>1900 ohms) across a ringer load that is at least 5REN. REN stands for Ringer Equivalent Number. It is a measurement of howmuch ringing power certain telephone equipment takes. REN numbers areused in the United States to designate how many pieces of telephonyequipment can be connected to the same subscriber line and still getthem ringing properly.

When the same ringing voltage specifications that were derived to meetthe specifications for the long loops are applied to heavy REN loads(e.g., 5C4A or 10K in parallel with 8 uF) with little or no loop inbetween the SLIC and the load, substantial SLIC power dissipation ispresent due to the smaller magnitude of the load impedance and due tothe phase shifted load current with respect to the drive voltage. Thephase shifted load current reaches its peak value when the load voltageis minimum and hence the drop across the SLIC is maximum. This conditionresults in significant SLIC power dissipation. This power dissipation isillustrated in the following equation:P _(SLIC) =P _(Battery) −P _(Load) +P _(CurSense),  (1)where P_(SLIC) is the average power dissipated in the SLIC, P_(Battery)is the power drawn from the battery, P_(Load) is the power dissipated inthe all of the load (i.e., telephony equipment and loop), andP_(CurSense) is the power dissipated in the current sense resistors ofthe SLIC.

Expanding the individual power components yields:

$\begin{matrix}{{P_{SLIC} = {\frac{2 \cdot V_{Bat} \cdot V_{Ring}}{\pi \cdot {Z_{OVL}}} - \frac{{\cos\left( {\angle\; Z_{OVL}} \right)} \cdot V_{Ring}^{2}}{2 \cdot {Z_{OVL}}} + \frac{R_{CurSense} \cdot V_{Ring}^{2}}{2 \cdot {Z_{OVL}}^{2}}}},} & (2)\end{matrix}$where V_(Bat) is equal to the total battery voltage applied across theSLIC (VBP−VBH), V_(Ring) is the peak ringing voltage generated by theSLIC, Z_(OVL) is the overall complex load including the internal currentsense resistors of the SLIC, fuse resistors, loop resistance, and theREN load, R_(CurSense) is the sum of the current sense resistors used inthe SLIC. Equation (2) is derived for a sinusoidal ringing waveformwithout any DC bias during ringing.

The first equation indicates that the power dissipated in the SLICshould be equal to power drawn from the battery, less the power that isdelivered to the load. The third term comes in to play because the SLIChas small current sense resistors that are used to measure the loadcurrent. Since these resistors also dissipate power, the SLIC powerincludes this term. The second equation indicates various powercomponents in terms the applied battery voltages, generated ringingvoltages, and the overall load impedance and the SLIC current senseresistor. Note that the P_(Load) component of the power would be zero ifthe phase shift introduced by the load circuit is 90 degrees. Thiscondition is approached for heavily reactive loads like 10K∥8 uF and5C4A REN loads. For these cases, the SLIC power increases as thegenerated ringing signal is increased. For resistive types of REN loads,the load power would have zero degree phase shift and hence more powerwould be delivered to the load, thus decreasing the power dissipated inthe SLIC.

As an example case, assuming V_(Bat)=VBP−VBH=90V−(−60V)=150V,Z_(OVL)=R_(CurSense) (36 ohms)+2*R_(Fuse)(50 ohms)+0 ohm loop+10K∥8 uFPretrip REN load, at 20 Hz ringing frequency (i.e.,Z_(OVL)=233.98−984.97j), V_(Ring)=130V peak ringing with no DC bias, thepower dissipated in the SLIC is P_(SLIC)=12.2623−1.929+0.2968=10.63 W.Hence, the SLIC is being asked to dissipate more than 10 W of power.Such a power dissipation would generate considerable amount of heat fromthe SLIC device. To remove the resulting heat generated from the SLIC atthis power would require additional surface area for better thermalconductivity, thereby reducing the density of the line card. Thiscondition works against the design objectives of lower cost and reducedoverall board size. Note that similar SLIC power dissipation conditionswould arise even for the 5C4A types of REN load as well under the sameconditions.

The ring-trip conditions present another challenge to the SLIC in termsof SLIC power dissipation. Some devices have a hardware analog currentlimit circuit that limits the loop current (e.g., around 100 mA). Such amechanism is employed to prevent the transformer magnetic coresaturation in the integrated voice and data (IVD) splitters and hence toprevent cyclic redundancy check (CRC) errors in the IVD and excessivecurrent draw from the power supply connected to the line car. Thecurrent limiting circuit acts when the subscriber goes off-hook whileringing or during typical DC feed conditions. The hardware current limitcircuit limits the loop current even before the firmware has had achance to react to detect the ring-trip. When the subscriber goesoff-hook while ringing the phone, in a short loop application, mostlikely the hardware current limit circuit will limit the load current.The hardware current limit circuit accomplishes limiting the loopcurrent by reducing the voltage applied across tip/ring leads. Assumingthe system has 136 ohms of fixed resistance (fuse and current senseresistors in the SLIC) and a 200 ohm off-hook resistance, the currentlimit circuit would limit the voltage across this load to the assumedcurrent limit of 100 mA times 336 ohms volts, unless of course thevoltage across the load is smaller than this voltage. So, while ringing,when the user goes off-hook the voltage across the load would be aclipped sine wave whose magnitude at the generator has a peak voltage of33.6V. If the battery applied made use of during ringing is 150V, theremaining 150V−33.6V+3.6V=120V is dropped across the SLIC. Thiscondition implies a SLIC power of 12 W under current limit conditions.Again, this level of SLIC power dissipation works against the designobjectives of lower cost and reduced overall board size.

This section of this document is intended to introduce various aspectsof art that may be related to various aspects of the disclosed subjectmatter described and/or claimed below. This section provides backgroundinformation to facilitate a better understanding of the various aspectsof the disclosed subject matter. It should be understood that thestatements in this section of this document are to be read in thislight, and not as admissions of prior art. The disclosed subject matteris directed to overcoming, or at least reducing the effects of, one ormore of the problems set forth above.

BRIEF SUMMARY

The following presents a simplified summary of the disclosed subjectmatter in order to provide a basic understanding of some aspects of thedisclosed subject matter. This summary is not an exhaustive overview ofthe disclosed subject matter. It is not intended to identify key orcritical elements of the disclosed subject matter or to delineate thescope of the disclosed subject matter. Its sole purpose is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is discussed later.

One aspect of the disclosed subject matter is seen in a method forcontrolling power dissipated in a subscriber line interface circuitwhich includes measuring a power parameter of the subscriber lineinterface circuit during a ringing cycle, comparing the measured powerparameter to a target power parameter and adjusting at least one ringingparameter of a ringing signal generated by the subscriber line interfacecircuit based on the comparison.

Another aspect of the disclosed subject matter is seen a line card whichincludes a subscriber line interface circuit operable to generate aringing signal, and a subscriber line audio-processing circuit operableto measure a power parameter of the subscriber line interface circuitduring a ringing cycle, compare the measured power parameter to a targetpower parameter, and adjust at least one ringing parameter of theringing signal based on the comparison.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosed subject matter will hereafter be described with referenceto the accompanying drawings, wherein like reference numerals denotelike elements, and:

FIG. 1 is a simplified block diagram of a line card in accordance withone illustrative embodiment of the present subject matter;

FIG. 2 is a simplified block diagram of a feedback control loop used forcontrolling the amplitude of a ringing signal generated by a subscriberline interface circuit in the line card of FIG. 1;

FIG. 3 is a diagram of a state machine for controlling the feedbackcontrol loop of FIG. 2;

FIG. 4 is a diagram of a ringing signal controlled by the feedbackcontrol loop of FIG. 2;

FIGS. 5A and 5B illustrate battery switching during the ringing cycleimplemented by the line card of FIG. 1; and

FIGS. 6 and 7 illustrate a reduced ringing signal generated by the linecard of FIG. 1 in response to the identification of a ring-tripprecursor.

While the disclosed subject matter is susceptible to variousmodifications and alternative forms, specific embodiments thereof havebeen shown by way of example in the drawings and are herein described indetail. It should be understood, however, that the description herein ofspecific embodiments is not intended to limit the disclosed subjectmatter to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the disclosed subject matter asdefined by the appended claims.

DETAILED DESCRIPTION

One or more specific embodiments of the disclosed subject matter will bedescribed below. It is specifically intended that the disclosed subjectmatter not be limited to the embodiments and illustrations containedherein, but include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims. It shouldbe appreciated that in the development of any such actualimplementation, as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure. Nothing in thisapplication is considered critical or essential to the disclosed subjectmatter unless explicitly indicated as being “critical” or “essential.”

The disclosed subject matter will now be described with reference to theattached figures. Various structures, systems and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the disclosed subject matter with details thatare well known to those skilled in the art. Nevertheless, the attacheddrawings are included to describe and explain illustrative examples ofthe disclosed subject matter. The words and phrases used herein shouldbe understood and interpreted to have a meaning consistent with theunderstanding of those words and phrases by those skilled in therelevant art. No special definition of a term or phrase, i.e., adefinition that is different from the ordinary and customary meaning asunderstood by those skilled in the art, is intended to be implied byconsistent usage of the term or phrase herein. To the extent that a termor phrase is intended to have a special meaning, i.e., a meaning otherthan that understood by skilled artisans, such a special definition willbe expressly set forth in the specification in a definitional mannerthat directly and unequivocally provides the special definition for theterm or phrase.

Referring now to the drawings wherein like reference numbers correspondto similar components throughout the several views and, specifically,referring to FIG. 1, the disclosed subject matter shall be described inthe context of a simplified block diagram of an exemplary line card 100.The line card 100 includes a subscriber line interface circuit (SLIC)110, a subscriber line audio-processing circuit (SLAC) 120, batteries130 (i.e., VBL, VBH, VBP), battery voltage sense circuitry 140, loopvoltage sense circuitry 150, and protection resistors 160, 170. The linecard 100 interfaces with a loop/load circuit 180 representing thesubscriber loop (i.e., telephony equipment at the customer premise andthe subscriber line). The SLIC 110 includes loop current sense andcurrent limit circuitry 112 for sensing the current output by the SLIC110 and to provide a hardware limiting of that current, as describedabove, battery selection circuitry 114 for selecting between the variousbatteries 130 (i.e., VBL, VBH, VBP) provided to the SLIC 110, ringingsignal drive circuitry 116 for driving the ringing signal, and thermalfault sensing circuitry 118 for outputting a signal responsive to thetemperature of the SLIC 110 exceeding a predetermined threshold. TheSLAC 120 includes a ringing power control unit 122 that acts to reducepower dissipated in the SLIC 110 during ringing events by controllingthe ringing voltage and/or by switching the batteries 130, a batteryselection control unit 124 for interfacing with the battery selectioncircuitry 114 of the SLIC 110 to designate which batteries 130 should beselected, a ringing signal generator 126 for providing the parameters ofthe ringing signal to the ringing signal drive circuitry 116 of the SLIC110, a current spike detector 127, and a power spike detector 128. In anactual implementation, the line card 100 may have multiple SLICs 110,SLACs 120, and associated circuitry to service a plurality of subscriberloops.

As will be described in greater detail below, the ringing power controlunit 122 acts to reduce power dissipated in the SLIC 110. For ease ofillustration and to avoid obscuring the present subject matter, only thecircuitry on the line card 100, SLIC 110, and SLAC 120 needed to supportthis functionality is illustrated in FIG. 1. Of course, in an actualimplementation, other functional units and circuitry may be provided forperforming other functions of the line card 100 as understood by thoseof ordinary skill in the art.

Referring back to Equations (1) and (2) above, the ringing power controlunit 122 acts to reduce power dissipated in the SLIC 110 during ringingevents to attempt to support the design objectives of lower cost andreduced overall board size. To evaluate the power dissipation conditionsillustrated above, Equation (2) may be solved for a given target SLIC110 power and an assumed load impedance. As the SLIC power equation is asecond order quadratic equation, each solution provides an answer. Ifthe roots of Equation (2) are complex in nature, it indicates that nolevel of ringing voltage within the supplied battery dissipates thetarget amount of SLIC power. So, under those conditions, the SLIC 110can safely generate a user programmed ringing voltage without resultingin a power dissipation that is greater than the target powerdissipation. When the roots of Equation (2) are real, any root whosemagnitude is higher than the battery voltage cannot physically begenerated and hence can be ignored. When the roots of the equations arereal and are within the battery voltage limits, for those ringingvoltages, the SLIC 110 would dissipate the target SLIC power.

The SLIC 110 is expected to deliver a specified ringing voltage forvarious loads and various loop lengths, without knowing in advance theload and loop conditions. Hence, an empirical solution to Equation (2)would only be applicable only to cases where the load and loopconditions are known. In such a case, the ringing voltage could be fixedby the SLAC 120, such that the power dissipation in the SLIC 110 matchesthe target power level. However, this solution would not be effective ifthe load (i.e., customer premises telephony device) or the loopconditions were changed. This solution would also cause a configurationcontrol problem, due to the constant need to reconfigure the line card100 if conditions were to change and for the initial customization atthe installation site.

To implement power control, the ringing power control unit 122 receivesmeasurements of various voltages and currents from the loop currentsense and current limit circuitry 112, battery voltage sense circuitry140, and loop voltage sense circuitry 150. Using these measuredparameters, the ringing power control unit 122 computes the average SLICpower. The measured average SLIC power is compared against a target SLICpower (i.e., that may be a programmable or default value) to control theringing voltage that is driven out by the SLIC 110.

Turning now to FIG. 2, a simplified block diagram of a feedback controlloop (FCL) 200 implemented by the ringing power control unit 122 forcontrolling the ringing voltage to limit the power dissipated in theSLIC 110. In general, the characteristics of the FCL 200 are:

-   -   the FCL has a small residual error such that the SLIC power        dissipation under steady state conditions should be close to the        target SLIC power;    -   the FCL does not generate a ringing voltage that is higher than        a user-specified ringing voltage;    -   the FCL outputs the user specified ringing voltage if the load        is such that the power dissipated in the SLIC 110 for the user        specified voltage and the given load and battery conditions is        less than the target SLIC power;    -   the FCL acts sufficiently fast when the load conditions change        (i.e., from a heavy REN load to a low REN load, or vice versa),        while at the same time having very little ripple in terms of        ringing amplitude variations during the steady state;    -   the FCL should be able to store and recall necessary information        such that the adaptation under a given set of conditions happen        once and not every ringing burst;    -   the ringing amplitude changes are be applied during zero crosses        of the ringing waveform such that no IVD spurii are introduced        on the line; and    -   the FCL is stable for all load and ringing and battery        conditions.

The FCL 200 includes a subtractor 210 operable to receive the powertarget specified by the system designer, P_(T), and the measured power,P_(M), to generate an power error signal. The forward path of the FCL200 connected to the subtractor 210 includes a gain unit 220 fordetermining the forward loop gain, G, of the FCL 200, a low pass filter230, a power to voltage conversion unit 240 for outputting a voltageerror signal corresponding to the power error signal, an adder 250 foradding the voltage error signal to the user programmed ring signalvoltage, V_(RingP), a limiting unit 260 for limiting the voltage to theuser programmed ring signal voltage, a ringing waveform generator 270for generating the ringing waveform, and current sensor 280 (e.g., theloop current sense and current limit circuitry 112 of FIG. 1) formeasuring the drive current supplied to the load represented by theoverall complex load 290 represented by Z_(OVL). In the feedback path, apower measurement module 295 uses the line current, I_(L), sensed by theloop current sense and current limit circuitry 112, the load voltage,V_(R), sensed by the loop voltage sense circuitry 150, and the batteryvoltage applied across the SLIC 110, V_(Bat), sensed by the batteryvoltage sense circuitry 140, to generate the power signal representingthe power dissipated in the SLIC 110.

The value of the target power may vary depending on the particulardesign considerations (e.g., device density and line card size). Oneconsideration when determining the appropriate target power is whetherthe ringing voltage associated with the selected target power is able tomeet or exceed the minimum ringing voltage required for various RENloads. An exemplary value for the target power is about 3.5 W. Themeasured power, P_(M), is the average SLIC power dissipated in the twopreceding ringing half cycles, which may be represented by the Equation:

$\begin{matrix}{P_{M} = {\frac{1}{T}\left( {{\int{{V_{Bat} \cdot {I_{L}}}{\mathbb{d}t}}} - {\int{{V_{R} \cdot I_{L}}{\mathbb{d}t}}} + \left( {{other}\mspace{14mu} S\; L\; I\; C\mspace{14mu}{power}\mspace{14mu}{dissipation}} \right)} \right)}} & (3)\end{matrix}$

As the terms in the above equation are instantaneous samples, theintegration is performed over the two preceding ringing half cycles,where T is the period of the ringing signal.

The gain of the gain unit 220 may be set to affect the dynamic responseof the FCL 200. In the illustrated embodiment, the gain unit 220 is setso that the overall loop gain is about 25. Hence, G is set to a value ofabout 500. The gain is set to a high value because the voltage across aresistor varies as a function of the square root of power dissipated inthat resistor. For complex loads, the relation is more complex. In anycase, the high value for G allows the correction voltage to swing widelyeven for small power differences between the measured and target SLIC110 power and can easily exceed the highest possible ringing voltagespecified. The gain thus allows for very small residual error in thesystem. In the illustrated embodiment, the low pass filter 230 is asingle pole low pass filter and is the dominant pole in the FCL 200. Itscorner frequency determines the dynamics of the feedback loop. Thecorner frequency of the low pass filter 230 is controlled between twovalues. When conditions arise for the ringing power control unit 122that would call for adjusting to a new load, to decrease the time toconvergence, the corner frequency is increased to about 0.5 Hz withouterasing the history present in the filter 230. At all other times thelow pass filter 230 employs a corner frequency of about 0.05 Hz. Theresponse of the FCL 200 may also be varied by changing the gain, G,instead of or in addition to the corner frequency. The gain of thevoltage conversion unit 240, which transforms a quantity that has theunits of the power to a quantity that has the units of voltage, isunity. The limiting unit 260 ensures that the FCL 200 does not produce aringing signal amplitude that is larger than what the user has specifiedand also ensures that the ringing voltage does not fall below a minimumthreshold voltage (e.g., 1 V peak). A condition by which the FCL 200might attempt to generate more ringing voltage than programmed amplitudewould occur if the actual SLIC power dissipation is less than the targetvalue. This response would be contrary to the goal of minimizing thepower dissipation, so the FCL 200 is only allowed to reduce the ringingvoltage to reduce the SLIC power dissipation to the target powerdissipation, not to increase the ringing voltage if the actual powerdissipation is less than the target. The ringing waveform generator 270generates the ringing waveform that is applied across the load 290. Theinputs to the ringing waveform generator 270 are the ringing frequencyand the peak amplitude. The ringing peak amplitude is altered during thezero cross phase of the ringing signal generation.

The ringing power control unit 122 also employs a state machine 300illustrated in FIG. 3 to accomplish the goals for the FCL 200 describedabove. The state machine 300 is responsible for enabling/disabling theFCL 200 and clocking the FCL 200 when it determines the FCL 200 needs torun. The state machine 300 clocks the FCL 200 at every zero cross of theringing waveform. Thus, the ringing amplitude control algorithm runs attwice the ringing frequency. The state machine 300 employs severaldetectors that are used to detect conditions when the FCL 200 should beswitched to fast convergence mode by changing the corner frequency ofthe low pass filter 230. These detectors make use of signals like changein user specified ringing amplitude, sudden change in the measured SLICpower (load change indication), etc. The state machine 300 also employshysteresis (e.g., about 0.5 W) and time-outs (e.g., about 500 ms) thatlimit the transitions between the fast and slow convergence modes. Thestate machine 300 also disables the FCL 200 adaptation algorithm whenany precursors to ring-trip have been identified (i.e., as described ingreater detail below) or the ring-trip itself has been detected, since aringing signal is not applied after a ring-trip. Allowing the FCL 200 torun during ring-trip would cause it to move away from the convergedanswer since during ring-trip, the SLIC 110 would be dissipating adifferent amount of power as compared to ringing.

The state machine 300 stores and recalls applicable parameters so thatthe FCL 200 does not have to converge on every ringing burst. Forexample, the state machine 300 stores the last ringing voltage appliedby the FCL 200 for a ringing event. When a subsequent ringing eventoccurs, the state machine 300 configures the ringing waveform generator270 to output the previously used ringing voltage. If the loadconditions have not changed, the FCL 200 would not need to furtheradjust the ringing voltage. However, if the load conditions had changedbetween ringing cycles, the FCL 200 would automatically reduce theringing voltage from the previous value if the power dissipated isgreater than the target or increase the ringing voltage to the maximumspecified ringing voltage if the power is less than the target power,and the previous ringing voltage is less than the maximum.

FIG. 4 illustrates the operation of the FCL 200 during a series ofexemplary ringing cycles. For ease of illustration, only the ringvoltage waveform 400 is shown. Starting from a reset at point 410, theFCL 200 reacts to the SLIC power being greater than the target power byreducing the ringing amplitude in the first few cycles of ringing in thefirst ringing burst 420 and converges at the ringing voltage 430 thatresults in the SLIC 110 dissipating the target power at point 440. Inthe subsequent ringing bursts 450, 460, the state machine 300 providesthe previous amplitude 430 to the FCL 200 so that it is substantiallyconverged at the starts of the ringing bursts 450, 460.

In the illustrated embodiment, the ringing power control unit 122 alsoreduces power dissipated in the SLIC 110 by changing the selection ofbatteries 130 to switch out those batteries that are not necessary tosupport the ringing signal. For example, the maximum batteryconfiguration is only needed at the peaks of the ringing signal. As theringing signal approaches the zero crossings, the actual voltagerequired is significantly less. Hence, the ringing power control unit122 cooperates with the battery selection control unit 124 to direct thebattery selection circuitry 114 in the SLIC 110 to select theappropriate batteries 130. In a simple example, the battery selectioncontrol unit 124 directs the battery selection circuitry 114 to switchfrom VBH to VBL during some portion of the ringing cycle depending onthe amplitude of the ringing voltage that is being generated and thebattery voltages themselves. In another example, both the positive andthe negative batteries 130 may be switched. In case of heavy REN loadscenarios where there is a substantial phase difference between theringing voltage and the ringing current, the SLIC 110 power dissipationsavings brought about due to battery switching can be significant.Simulation results have shown that about 0.5 to 0.75 W SLIC 110 powersavings can be achieved for typical ringing scenarios, which issignificant when compared to a total target power dissipation ofapproximately 3-4 W.

FIGS. 5A and 5B illustrate the battery switching cycle employed by thebattery selection circuitry 114 at the direction of the batteryselection control unit 124. In FIG. 5A, the battery selection controlunit 124 cycles between the negative batteries (i.e., VBL or VBH),during the ringing cycle, and in FIG. 5B, the battery selection controlunit 124 switches both the positive battery (i.e., VBP or ground) andthe negative batteries (i.e., VBL or VBH).

When highly reactive REN loads (e.g., 10K∥8 uF or 5C4A—British Telecomringer load specification), there can be substantial phase shift in theload current. In other words, as the ringing voltage across the load isdecreasing, the current through the load increases due to the phaseshift between the voltage and current in the highly reactive loads. Notethat as the voltage across the load is decreasing, the voltage acrossthe SLIC 110 is increasing, because the voltage drop across the SLIC 110is the power supply minus the load voltage. The increased SLIC 110voltage drop and increased load current results in increased SLIC 110power dissipation.

The battery selection control unit 124 employs battery switching duringringing because when the voltage across the load is smaller, only asmaller power supply is required. So, when the voltage signals drivenout on the tip and ring leads are such that they can be supported with asmaller available battery, then the battery switching may be performed.Hence, whether or not battery switching can take place depends on theringing signal generation configuration, which includes the ringing DCbias, the ringing sinusoidal signal amplitude, and the battery voltages.

The battery selection control unit 124 generates the control signalsnecessary for the battery selection circuitry 114 to make the batteryselections. There are various factors that are considered whenimplementing the switching logic. There may be delays involved inissuing or acting on the battery switching command with respect to theringing waveform generation signal path. There may be also delaysinvolved in implementing the battery switching once the command isreceived. Delays may also be introduced by external or internal filtersthat are present between the ringing signal generation (i.e., in thesoftware) and the final power amplifier. These delays could cause signalclipping if the switching does not account for them. Headroom factorsmay be employed in the switching so that the switching does not occur atthe exact battery thresholds, but rather only after the threshold andthe headroom factor have been met.

The following pseudo-code represents the logic employed by the batteryselection control unit 124 for battery switching in one embodiment:

  % Postive Battery Selection Logic   positiveSelectedBat(i) = vbp; %Assume VBP is needed   if  (vTipAbs(i)  +  posBatterSwitchHeadRoom  <= 0)  &&  (vRingAbs(i)  +     posBatterSwitchHeadRoom <= 0)      positiveSelectedBat(i) = 0; % No need of VBP battery   end   %Negative Battery Selection Logic   negativeSelectedBat(i) = vbh; %Assume VBH is needed   if  (vTipAbs(i)  −  negBatterSwitchHeadRoom  >= vbl)  &&  (vRingAbs(i)  −     negBatterSwitchHeadRoom >= vbl)      negativeSelectedBat(i) = vbl; % No need of VBH battery   end where  vTipAbs(i) and vRingAbs(i) are the instantaneous Tip and Ring leadvoltages     that are being driven out by the SLIC 110;  posBatterSwitchHeadRoom  and  negBatterSwitchHeadRoom  are  voltage    headrooms provided to prevent signal clipping in the positive andthe     negative batteries;   vbp,  vbl,  vbh  are  the  positive,  low negative,  and  most  negative  battery     voltages; and  positiveSelectedBat(i)  and  negativeSelectedBat(i)  are  the variables  that     contain the selected battery voltages.

One technique for addressing the delays may be implemented by thebattery selection control unit 124 by operating on the data samples thatwill be generated in a future time period. The horizon for the futuretime period may be selected based on the worst case delays expected inthe system when selecting the higher battery (VBP or VBH) and by makinguse of the future and the present samples when selecting the lowerbattery (VBL or ground).

Another approach is to compute the worst case headroom increment factorsreflected by the posBatterSwitchHeadRoom and negBatterSwitchHeadRoomvariables to account for the delays. In one embodiment, the worst caseheadroom factor may be computed as a function of the phase at which thebattery switching is expected to happen and, at that phase, the rate ofchange of the ringing voltage that is being generated. Using the phaseat which battery switching is expected to happen, the battery selectioncontrol unit 124 can calculate how much the ringing voltage will changeat that phase in a given time interval based on the worst case delay inthe system. The computed voltage change can then be added to the fixedheadroom computation.

Another significant factor associated with SLIC 110 power dissipation isthe power dissipated during a ring-trip event. A ring-trip occurs whenthe telephony device goes off hook during a ringing cycle. Theidentification of the ring-trip terminates the ringing cycle, and thepositive battery is removed. However, due to various requirementsimposed by telecommunications standards to prevent spurious ring-tripindications, there is typically a delay between when the device goes offhook and the ring-trip is identified to allow debouncing of the off-hookevent.

To reduce power dissipation in the SLIC 110 during a ring-trip, theringing power control unit 122 attempts to identify a ring-trip inadvance of the actual ring-trip detection. To that end, the ringingpower control unit 122 identifies a ring-trip precursor indicator thatsuggests that a ring-trip is imminent. Exemplary ring-trip precursorsinclude a current spike precursor, a power spike precursor, or a thermalfault precursor.

The current spike detector 127 is conventionally used to address the 200ohm 12 ms no ring-trip, pre-trip load condition. The current spikedetector 127 compares the instantaneous samples of the load current(e.g., running at a 2 KHz rate) to determine if the load currentdifference between the current samples is more than a user programmedthreshold. Once such a condition is found, the ring-trip detectionalgorithm is disabled for 16 ms. The threshold of the current spikedetector 127 may be set such that the detector 127 detects theapplication of a 200 ohm load at any phase of the ringing cycle. Theringing power control unit 122 interfaces with the current spikedetector 127 to identify a ring-trip precursor when the threshold ismet. Hence, when the current spike detector 127 detects a current spikeand disables the ring-trip detection, the ringing power control unit 122indicates a ring-trip precursor event.

Upon going off-hook, due to the nature of current limit circuitry 112,the power dissipation is the SLIC 110 jumps. The current may jump evenif the current limit is not reached due to the rise in current. Thiscurrent jump in the SLIC 110 may be used by the ringing power controlunit 122 as another ring-trip precursor indicator. A power spikering-trip precursor may be useful in markets where the 200 ohm 12 mspre-trip condition is not applicable or when the current spike detector127 fails to detect a current spike. The power spike detector 128 usesthe SLIC 110 power dissipation as measured by Equation (3) above.Assuming the applied stimulus signal (i.e., ringing signal) is changingsmoothly, the load circuit is unchanging, and there are nodiscontinuities in the transfer functions of various elements in thesystem, the derivative of the measured SLIC power should always be lessthan a predetermined quantify. The highest expected derivative can becomputed for a given set of conditions. When a user picks up the phone(i.e., ring-trip) there is a discontinuous change in the system thatcauses the SLIC 110 power to jump suddenly. This jump could be furtherincreased due to the action of the current limit circuitry 112 in theSLIC 110. Such a power jump would be pronounced in short and mediumlength loops.

The power spike detector 128 monitors the power difference between twosuccessive SLIC power samples. Based on various simulations, the worstcase power spike that can be expected under steady state ringingoperations is less the 3 W. So, for the power spike detector 128, athreshold of about 3 W may be used to identify a ring-trip precursor.

A third ring-trip precursor detection technique employs the thermalfault sensing circuitry 118 of the SLIC 110. For example, the thermalfault sensing circuitry 118 may indicate a thermal fault in response tothe temperature of the SLIC 110 exceeding a predetermined threshold(e.g., 180 degrees C.). The thermal fault sensing circuitry 118 isconventionally used to protect the SLIC 110 under fault conditions. In atypical deployment scenario, where in the system is placed close to theneighborhood without air conditioning, it is not unlikely that the linecard 100 could be at a high temperature (i.e., as high as 85 degreesC.). Given the SLIC 110 packages and the thermal conductivity betweenthe die of the SLIC 110 and the SLIC package, it is possible that theSLIC 110 die temperature could raise as much as 25 degrees C. per Wattof SLIC 110 power dissipation. Given the small thermal mass of the SLIC110, the temperature of the SLIC 110 may raise very quickly. Hence, inresponse to a user picking up the phone, it is not unlikely that onewill see a thermal fault. This is especially true if the ambienttemperature is high and the SLIC 110 exhibits a power dissipation ofabout 3 W during ringing. The thermal fault event that indicates thering-trip precursor, and any further thermal further faults that occur,are not reported to the SLAC 120 host processor until about at least 30ms have elapsed. Only thermal faults that still persist after thistime-out are reported to the host.

In response to identifying a ring-trip precursor, the ringing powercontrol unit 122 initiates a reduced ringing state that does not makeuse of the positive battery, VBP. During the reduced ringing state, theringing signal generator 126 applies a DC bias with a magnitude anddirection the same as specified by the user in the ringing profile, butcentered around VBH/2 instead of being located at the user programmedvalue. The ringing amplitude during the reduced ringing state is equalto:|VBH|−|DC Bias|−overhead voltage,  (4)where the overhead voltage is about set at an implementation-specificvalue to avoid clipping (e.g., 8 V).

Upon identifying a ring-trip precursor event, the ringing power controlunit 122 directs the ringing signal generator 126 to ramp from thepresent tip/ring voltages to the DC bias voltages before re-applying thereduced ringing signal. The ramp is started from the present voltages onthe line is because the drive voltages might have been modified due tothe action of the current limit circuit and in the interest of reducingthe time of continued use of the positive battery. The ramping alsoprevents IVD glitches due to a sudden jump. While the ramp is beingimplemented and when the drive voltages have dropped to a voltage levelthat does not need the support of the positive battery, VBP, the usageof the positive battery is stopped, which significantly reduces thepower load on the SLIC 110.

FIG. 6 illustrates the behavior during the reduced ringing state. Theoff-hook condition is applied at points 600 where the tip/ring voltagessuddenly collapse due to the action of the current limit circuit. Thedetection of a ring-trip precursor at points 610 triggers the DC biasramps 620 in the ring and tip voltages and the positive battery isremoved at point 630 during the ramps 620. The transition to the reducedringing state occurs at points 640.

The telephony device may go back on-hook after the ring-trip precursorevent is detected without the ring-trip itself being found. Such ascenario happens when testing for the 200 ohms 12 ms pre-trip loadcondition, where the load is removed even before the ramp is complete(i.e., which is likely to take up to 16 to 20 ms depending on theinstant in time the off-hook was applied). In such cases, the ringingpower control unit 122 terminates the reduced ringing. To detect theoff-hook removal condition the ringing power control unit 122 has twomechanisms, a time-out based mechanism, and a load power mechanism.

Based on the operation of the ring-trip modules, the ring-trip eventmust be found within two ringing cycles once the reduced ringing signalis applied. If the ring-trip event has not been found after this time,the ringing power control unit 122 concludes that the telephony devicehas gone back on-hook and the full ringing signal is restored.

The second approach involves looking at the power that is beingdelivered to the load 180. If the power delivered to the load is minimal(e.g., less than 0.1 W) it indicates the absence of a resistive load,and hence, the device is on-hook. To estimate load power, instantaneoussamples of the load power measurements are averaged for one completeringing cycle.P _(L) =V _(L)(inst)·I _(L)(inst),  (5)

If a return to full ringing voltage is indicated by the power thresholdtechnique or the timeout technique, the full ringing signal isreapplied. The return to the full ringing mode of operation starts withramping from the DC biases used in the reduced ringing state back tothose made use of in the full ringing state, as illustrated in FIG. 7,which shows the expected behavior during the 200 ohms 12 ms pre-tripload test.

The ring-trip precursor is identified at points 700, and the biasramping is initiated. The load that caused the ring-trip precursor isremoved by the time the reduced ringing state starts at points 710.While in the reduced ringing state, the ringing power control unit 122determines that the load power is minimal and the full ringing signal isrestored by initiating the ramp to the normal DC bias at points 720, andresuming the normal ringing signal at points 730.

Even with the reduction in ringing amplitude due to the FCL 200 and thereduced ringing amplitude state, the ring-trip would still be detectedin a timely fashion using a conventional ring-trip detector. The DCringing bias that was applied to the ringing was not modified other thanby relocating it in the reduced ringing state. Therefore, the long loopdetector continues to work as expected. However, with a short loopring-trip detector, the standard short loop ring-trip threshold setequal to the programmed ringing voltage divided by the minimum of allthe pre-trip REN loads that the system is expected to serve. Under heavyREN load conditions, if the SLIC power dissipation exceeds theprogrammed threshold, the FCL 200 would have reduced the ringingamplitude, thereby decreasing the RMS value of the load current thatwould be observed in the line upon a ring-trip. To compensate the shortloop ring-trip detector, the user programmed short loop ring tripparameters are multiplied by the factor:

$\begin{matrix}{\frac{{Current}\mspace{14mu}{Ringing}\mspace{14mu}{Amplitude}\mspace{14mu}{Applied}}{{User}\text{-}{specified}\mspace{14mu}{Ringing}\mspace{14mu}{Amplitude}}.} & (5)\end{matrix}$

By applying the scaling factor, the threshold is compensated for theringing amplitude reduction caused by the FCL 200 or the reduced ringingand ring-trip events may be detected by the short loop detector in thefull-ringing or the reduced-ringing states.

Controlling power dissipation during the ringing cycle, by adaptivecontrolling ringing amplitude, performing battery switching, oremploying a reduced ringing cycle after identifying ring-trip precursorssignificantly reduces the heat generated by the SLIC 110. Reducing theheat generated allows greater device densities on the line card 100,thereby reducing the cost/per subscriber line.

The particular embodiments disclosed above are illustrative only, as thedisclosed subject matter may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of thedisclosed subject matter. Accordingly, the protection sought herein isas set forth in the claims below.

1. A method for controlling power dissipated in a subscriber lineinterface circuit, comprising: measuring a power parameter of thesubscriber line interface circuit multiple times over a plurality ofcycles of a ringing burst, the power parameter indicating average powerdissipated in the subscriber line interface circuit; comparing themeasured power parameter at each measurement to a target power parameterfor average power dissipated in the subscriber line interface circuit;and adjusting at least one ringing parameter of a ringing signalgenerated by the subscriber line interface circuit over the plurality ofcycles of the ringing burst based on the comparisons.
 2. The method ofclaim 1, further comprising implementing a feedback control loopoperable to receive the measured power parameter and the target powerparameter and generate an error signal based thereon for adjusting theringing parameter.
 3. The method of claim 2, wherein implementing thefeedback control loop comprises filtering the error signal using a lowpass filter.
 4. The method of claim 3, wherein the low pass filter is asingle pole filter having a predetermined corner frequency, and themethod further comprises adjusting the corner frequency responsive to achange in the measured power parameter greater than a predeterminedthreshold.
 5. The method of claim 3, wherein the low pass filter is asingle pole filter having a predetermined corner frequency, and themethod further comprises adjusting the corner frequency responsive tochanging a maximum value for the ringing parameter.
 6. The method ofclaim 2, wherein implementing the feedback control loop furthercomprises limiting the at least one ringing parameter based on a maximumthreshold for the ringing parameter.
 7. The method of claim 2, furthercomprising: establishing a steady state value for the ringing parameterduring the ringing cycle; and employing the steady state value as aninitial value for providing the ringing signal during a subsequentringing cycle.
 8. The method of claim 2, wherein the error signalcomprises a power error signal, and implementing the feedback controlloop further comprises: converting the power error signal to a voltageerror signal; and adding the voltage error signal to a maximum value forthe ringing parameter to generate the ringing parameter.
 9. The methodof claim 1, wherein the ringing parameter comprises an amplitude of theringing signal.
 10. The method of claim 1, wherein measuring the powerparameter comprises: measuring a current supplied by the subscriber lineinterface circuit; measuring a voltage applied across the subscriberline; measuring a battery voltage provided to the subscriber lineinterface circuit; and determining the power parameter based on themeasured current, the measured voltage applied across the subscriberline, the measured battery voltage, and the ringing parameter.
 11. Themethod of claim 1, wherein a plurality of batteries are coupled to thesubscriber line interface circuit, and the method further comprisesswitching selected batteries during the ringing cycle based on values ofthe ringing signal during the ringing cycle.
 12. The method of claim 11,wherein the plurality of batteries includes at least a high negativebattery and a low negative battery, and the method further comprisesswitching between the high negative battery and the low negative batterybased on the values of the ringing signal.
 13. The method of claim 11,wherein the plurality of batteries includes at least a positive battery,and the method further comprises switching between the positive batteryand ground based on the values of the ringing signal.
 14. A line card,comprising: a subscriber line interface circuit operable to generate aringing signal; and a subscriber line audio-processing circuit operableto measure a power parameter of the subscriber line interface circuitmultiple times over a plurality of cycles of a ringing burst, the powerparameter indicating average power dissipated in the subscriber lineinterface circuit, compare the measured power parameter at eachmeasurement to a target power parameter for average power dissipated inthe subscriber line interface circuit, and adjust at least one ringingparameter of the ringing signal over the plurality of cycles of theringing burst based on the comparisons.
 15. The line card of claim 14,wherein the subscriber line audio-processing circuit includes a ringingpower control unit operable to implement a feedback control loop toreceive the measured power parameter and the target power parameter andgenerate an error signal based thereon for adjusting the ringingparameter.
 16. The line card of claim 15, the feedback control loopcomprises a low pass filter operable to filter the error signal.
 17. Theline card of claim 16, wherein the low pass filter is a single polefilter having a predetermined corner frequency, and the ringing powercontrol unit is operable to adjust the corner frequency responsive to achange in the measured power parameter greater than a predeterminedthreshold.
 18. The line card of claim 16, wherein the low pass filter isa single pole filter having a predetermined corner frequency, and theringing power control unit is operable to adjust the corner frequencyresponsive to a change in a maximum value for the ringing parameter. 19.The line card of claim 15, wherein the feedback control loop furthercomprises a limiting unit operable to limit the at least one ringingparameter based on a maximum threshold for the ringing parameter. 20.The line card of claim 15, further comprising a state machine operableto control the feedback control loop, store a steady state value for theringing parameter during the ringing cycle, and provide the storedsteady state value as an initial value for the feedback control loopduring a subsequent ringing cycle.
 21. The line card of claim 15,wherein the error signal comprises a power error signal, and thefeedback control loop further comprises a conversion unit operable toconvert the power error signal to a voltage error signal.
 22. The linecard of claim 14, wherein the ringing parameter comprises an amplitudeof the ringing signal.
 23. The line card of claim 14, furthercomprising: a plurality of batteries operable to provide a batteryvoltage signal to the subscriber line interface circuit; battery voltagesense circuitry operable to measure the battery voltage signal; loopvoltage sense circuitry operable to measure a voltage applied across thesubscriber line; current sensing circuitry in the subscriber lineinterface circuit operable to measure a current supplied by thesubscriber line interface circuit; a ringing signal generator in thesubscriber line audio-processing circuit operable to generate theringing signal using the ringing parameter, wherein the subscriber lineaudio-processing circuit is operable to determine the power parameterbased on the measured current, the measured voltage applied across thesubscriber line, the measured battery voltage, and the ringingparameter.
 24. The line card of claim 14, further comprising a pluralityof batteries coupled to the subscriber line interface circuit, thesubscriber line interface circuit comprises battery selection circuitryoperable to select individual ones of the batteries, and the subscriberline audio-processing circuit includes a battery selection control unitoperable to interface with the battery selection circuitry to switch theselected batteries during the ringing cycle based on values of theringing signal during the ringing cycle.
 25. The line card of claim 24,wherein the plurality of batteries includes at least a high negativebattery and a low negative battery, and the battery selection controlunit is operable to switch between the high negative battery and the lownegative battery based on the value of the ringing signal.
 26. The linecard of claim 24, wherein the plurality of batteries includes at least apositive battery, and the battery selection control unit is operable toswitch between the positive battery and ground based on the value of theringing signal.
 27. The method of claim 1, wherein adjusting the atleast one ringing parameter comprises adjusting the at least one ringingparameter during a zero crossing of the ringing signal.
 28. The linecard of claim 14, wherein the a subscriber line audio-processing circuitis operable to adjust the at least one ringing parameter during a zerocrossing of the ringing signal.